Hardening and Tempering Colors

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by Jan-Eric Nyström

Just for those who have asked for it, here's a quick picture of the colors to look for when heating iron to harden & temper it:

Hardening and Tempering Color Chart, by Jan-Eric Nyström. Used with permission.

Temperature conversion for the colors listed above:

  • 600°C = 1112°F
  • 700°C = 1292°F
  • 800°C = 1472°F
  • 900°C = 1652°F
  • 1000°C = 1832°F
  • 1100°C = 2012°F

I shot a piece of bright iron heated from one end to show the tempering colors. My source (a book from 1943) gives these temperatures for certain purposes:

  • 230°C (446°F), light straw/yellow: Very hard cutting tools, engraving tools. Brittle.
  • 260°C (500°F), dark yellow to brownish red: Lathe tools
  • 270°C (518°F), reddish brown: drills, screwcutting taps
  • 290°C (554°F), blue: springs, tools for stone and wood